Conventional filtering systems for filtration of solids and oils generally use one or both of two common methods to remove the layer of accumulated particles from the outside surface of the filter element at the end of the filtration cycle. One of these methods involves a high pressure pulse of compressed air introduced after cake drying on the downstream side of the filter and caused to flow in a direction opposite to the filtrate flow direction. A second method utilizes abrupt changes in the speed and direction of the filter element and attached cake by multiple rapping or vibration in hope of overcoming the adhesive forces through a change in cake momentum over a very short time interval.
The standards of the industry thus far examined have attempted to evoke this second method by moving the entire supporting structure of the filter element, the supporting structure usually having a mass very large in comparison to that of the filter element. The downfall of this approach seems to be that the large mass limits the acceleration of the support structure, and the material strain when attempting to halt the motion of such a large mass lengthens the time interval over which the final velocity change takes place, resulting in lower forces being produced in the adhesive layer and insufficient cake removal. Other obvious disadvantages of this approach are noise pollution and the wear and associated replacement costs of very large parts.
Tests were conducted on a candle filter comprising a filter screen usually coated with a filter aid or media and utilizing an air blow method of filter cake discharge. The filter cake, consisting typically of an initially deposited layer of diatomaceous earth and a subsequent, thin, outer layer of clay, was subjected to final differential pressures ranging from 16 to 40 p.s.i. for a time period ranging from 15 minutes to 45 minutes during the filtration cycle. A pulse of air with a flow rate of 400 cfm @ 100 p.s.i. was then directed through the cake in the reverse flow direction. In no cases was the cake totally removed and the non-repeatability of test results was apparent. It was believed that random distribution and orientation of high pressure induced cracks in the cake produced inconsistent and undesirable results. Tests were also conducted on a prior art mechanical rapping device and these were found to be unsuccessful after multiple raps, e.g. as many as 45 raps, for the conditions tested.
Heretofor, others have developed various rapping devices for discharging filter cake from a filter. These are, in part, exemplified by U.S. Pat. No. 3,100,190 in which a candle mechanism connected to a spring shakes a series of vertical filter tubes connected to a common partition up and down against a fixed flange extending inwardly from a filter tank wall; 3,482,700 in which a movable horizontal partition is bounced up and down by an actuator with springs in the filter chamber; 4,265,771 in which a rapper moves a partition carrying a series of tubes and where the partition sealingly abuts and is movable relative to the filter casing; and 4,526,688 and 4,289,630 where a vibratory impactor imparts vertically directed shock waves to a rigid manifold from which filter tubes fixedly depend. In 4,741,841 a vibrator continuously vibrates a series of tubes mounted on a flexible diaphragm and in 4,517,086 a rapper has a vertical plunger which shakes a partition.